The invention relates to a high-pressure gas discharge lamp comprising:
a lamp vessel which is closed in a vacuum-tight manner and which has a quartz glass wall enclosing a discharge space; PA1 metal foils embedded in the wall of the lamp vessel and each connected to a respective external current conductor; PA1 tungsten electrode rods which may have an additive, are connected each to a respective one of said metal foils, and project from the wall of the lamp vessel into the discharge space; and PA1 an ionizable filling in the discharge space.
Such a high-pressure gas discharge lamp is known from U.S. Pat. No. 4,594,529. The known lamp is suitable for use as a vehicle headlamp and has electrode rods which may or may not have an enveloping winding at their ends and which may be made of, for example, thoriated tungsten.
High requirements are imposed on the speed with which the lamp, after it has been energized, provides a major fraction of the luminous flux which it provides during stable operation. It is also necessary for the lamp to be capable of being ignited while it is still hot owing to a previous operating period. The lamp is ignited at a voltage of several kV and several kHz in order to comply with these requirements.
It was found that the known lamp has only a short life when it is frequently switched on and switched off again after a short operating period. The lamp vessel is then found to have become leaky, owing to which filling components have escaped and the lamp no longer ignites, or the lamp vessel is then cracked.
In the manufacture of the known lamp, a seal is made in which one or several said metal foils are enclosed in the wall. During this, the quartz glass is softened at the area where this seal is to be created in the presence of the metal foil, the external current conductor and the electrode rod. After forming the seal the glass is allowed to cool down. Owing to its comparatively high coefficient of linear thermal expansion (approximately 45 * 10.sup.-7 K.sup.-1), the electrode rod then contracts more strongly than does the quartz glass, glass having an SiO.sub.2 content of at least 98% by weight (approximately 6.times.10.sup.-7 K.sup.-1) in which it is embedded. This creates a capillary space around the electrode rod. No such capillary space is created around the metal foil, often a molybdenum foil, because of the foil shape.
When the known lamp is ignited, the temperature of the electrode rods rises steeply owing to the high current flowing through them and owing to heat transfer from the discharge. The quartz glass does not instantaneously follow this temperature rise. Owing to their higher temperature and their higher coefficient of expansion, the rods will come into contact with the quartz glass and exert pressure on it. This pressure creates microcracks, in the quartz glass, which microcracks may increase in number and size during subsequent ignition periods and lead to lamp leaks.
A possibility of avoiding these effects consists in the creation of a comparatively wide space around the electrode rods, so that it is prevented that the glass of the wall touches the rods during the manufacture of the seal. This possibility, however, is not always feasible in practice, because a space of comparatively low temperature arises thereby during operation, in which space filling components of low volatility may accumulate and thus may no longer take part in the discharge. Lamps having such a comparatively wide, but restricted space around the electrode rods are disclosed in EP 0 206 598-B1.
U.S. Pat. No. 3,868,528-A discloses a metal halide lamp in which current supply conductors for a main and for an auxiliary electrode are enclosed in a seal of the lamp vessel next to one another. Under the influence of their opposite potentials and the metal halide, devitrification of the seal may occur in this lamp. To prevent this, the spaces surrounding the electrode rods in this lamp are filled with an alkaline earth-aluminosilicate glass with a comparatively low melting point and with a linear thermal coefficient of expansion which is close to that of tungsten. A disadvantage of this is that the electrode rod must be kept above the metal foil while the seal is being made in order to allow the silicate glass to flow into the space around the rod and to keep it there. It is not possible in this position to cool an ionizable filling in the lamp vessel sufficiently. This filling must be provided in a subsequent step through an exhaust tube, after which the exhaust tube is sealed. An exhaust tube, however, interferes with the beam paths of generated light. Accordingly, the resulting lamp is not suitable for reflector applications where precise beam patterns are requested, such as in automotive lamps.
The invention has for its object to provide a high-pressure gas discharge lamp of the kind mentioned in the opening paragraph which is of a simple construction and in which premature failure is counteracted.